1. Technical Field
This invention relates to a blow-molded container and, in particular, to a container having a small blow ratio and molded by a direct blow molding process.
This invention also relates to a mold for use in blow molding, and in particular, to a mold for use in blow molding in which flash development is controllable at the bottom pinch-off portion of the containers, such as bottles.
2. Background Art
As shown in FIG. 20, an extruder 121 is used in an ordinary direct blow molding process to extrude parison P, which is a molten resin in a cylindrical shape. This parison P is then put between separated two halves of a split mold 101 to be used in blow molding, and is pinched in the mold clamping process. A pair of pinch-off blades is disposed at the bottom pinch-off section 105 of the cavity 102 in the lower portion of the split mold 101 and is used to cut off the parison at its lower end. The cut end of the parison is sealed in situ by thermal welding. A parison cutter 122 cuts the upper portion of the cylindrical molten resin to form a parison P in the bottomed cylindrical shape. Then, an air nozzle is inserted into the parison P from the head above the split mold 101 to blow air into the parison P and to blow-mold it into a molded product.
FIG. 6 shows an example of a conventional blow-molded container. FIGS. 7 and 8 show the rear half of a conventional split mold and a conventional mold-clamping and pinch-off process, respectively. In its structure, the split mold 101 has front and rear halves that have been parted on one plane (mold parting face 111). FIG. 8(c) shows a cross-sectional plan view of the split mold 101 in the closed state. As shown, the pinch-off line 114 is formed by the pinch-off blades 106 on the bottom surface 104 of the bottom 103 of the mold cavity 102, and is also located along the mold parting line 113, which is formed by the mold parting face 111.
The length of the pinch-off section 105, i.e., the lateral length of the pinch-off blades 106, is about 1.6 Dp, which is the width of the parison measured when the parison with an outer diameter of 1 Dp is pressed flat. As a result, the pinch-off line 14 with a length of about 1.6 Dp is formed on the same line as the mold parting line 13 on the underside surface of the molded container. (See FIG. 6.)
Conventionally, various improvements have been made for the structure of the container near the bottom pinch-off portion to increase the strength of the bottom seal of the molded container and from the point of view of outer appearances. Japanese Application No. 1995-88943, for example, describes a method of increasing the strength of bottom seal of, above all, a laminated container, by forming uneven pushing dents on both sides of the container pinch-off portion. Japanese Application No. 1997-262902 describes a method of controlling bottom seal thickness by disposing a compression plate at a position below the pinch-off section of a molding tool and allowing the compression plate to pinch the resin that runs off from the pinch-off section (hereinafter referred to as flash).
As described above, a pinch-off line 1.6 Dp in length is usually formed on the underside surface of the container, which is a direct blow-molded product. Because the pinch-off line thus formed does not exceed the periphery of the underside surface in the case of a container having a sufficiently large blow ratio, the containers are utilized in various applications with no problem of outer appearances. However, when the container having a small blow ratio is blow-molded, the container pinch-off portion would have a length close to or equal to the diameter of the bottom surface of the mold cavity. During the pinch-off process, the parison is pressed flat at the bottom of the mold cavity. At that time, the parison deforms laterally, or the parison resin flows laterally from the right and left sides of the mold pinch-off section toward the mold parting face located to the right and left. As a result, the resin that has run off remains not only on the underside bottom of the container, but also in the vicinity of the mold parting lines on the sidewall of the body right above the bottom. Thus, the so-called flash is formed, and the flash lines remain on the container product.
The length of the container pinch-off portion can be made smaller than the diameter of the underside surface of the container if the container has a blow ratio close to 2. Even in that case, it happens that, once the parison has been pinched and pressed flat by the mold pinch-off section, the resin may flow from the right and left sides of the pinch-off section laterally along the mold parting face. Such flows of the resin leave flash lines nearby the mold parting lines on the body wall right above the bottom surface of the molded container. Even if there has been no flash line, there still would remain a glossy spot or a color spot, including a drawing spot, an orientation spot, and/or a cooling spot, which are the factors caused by the changes in the resin flow.
Furthermore, if it is necessary to thicken the parison wall, those glossy or color spots as described above would develop even at a blow ratio close to 3.
Many small containers having a blow ratio of 1.6 or less are used in the field of bottles for cosmetic materials, such as mascara, eye liners, etc., and this field is where the outer appearances of the container are important factors affecting marketability of products. However, in the case of a container with a small blow ratio, the available length of the container pinch-off portion is up to the diameter of the mold cavity at the maximum. Thus, the pinch-off section can have a length less than 1.6 Dp, which is the width of the parison that has been pressed flat. In that case, the parison is pressed not only at the mold pinch-off section, but also on the mold parting face nearby the pinch-off section. The flash lines inevitably develop in the vicinity of the mold parting lines on both sides of the body right above the bottom.
These flash line, glossy spot, color spot, and the like are a large disfiguring problem concerned with the marketability of products especially in the case of cosmetic containers. Up to now, any solution has never been proposed to solve this problem. Even if post-treatment is used to improve outer appearances of the containers, it takes a long time of processing.
This invention has been made to solve the above-described technical problem found in conventional art. The technical problem of this invention is to inhibit the flash development on the underside bottom surface of the container molded by the direct blow method, especially in the lateral direction from the right and left ends of the mold pinch-off section. An object of this invention is to provide a blow-molded container having a low blow ratio and good appearances with no flash line, glossy spot, and/or color spot on the sidewall of the container near the bottom.
Another object of this invention is to create a mold for use in blow molding, which can provide a blow-molded container having a low blow ratio and good appearances with no flash line, glossy spot, and/or color spot on the sidewall of the container near the bottom.